The topic of transversal roof bracing design was discussed. The analysis was performed in ABAQUS for a part of the steel single-storey building roof consisting of truss girders and purlins. Structure with both diaphragm bracing (trapezoidal sheeting) and bar bracing was considered. Imperfection-origin forces (implemented using stabilising force) and wind loads were taken into account. Following modifications of the numerical model were analysed: substitution of the 3D shell model of the sheeting with an equivalent orthotropic shell, omission of the elasticity properties of the purlin-to-sheeting connection, omission of the eccentricity between purlins and sheeting. Influence of the simplifications of the numerical model on forces in bracing was assessed. Substitution of the 3D shell model of the sheeting with the equivalent orthotropic shell model with modification of the matrix stiffness according to the stressed skin theory gave more satisfactory results of the extreme forces in bar bracing than using “standard” stiffness matrix element values. What is more, the numeration simplification of the purlin-to-sheeting connection (omission of the connection flexibilities) affected the results less than the omission of the purlin-sheeting eccentricity.